Rock Mass Strength And Deformability Behaviour Of Sabaloka Igneous Complex

Abstract

Sabaloka plateau is located, 80 km north of Khartoum particularly between latitudes160 10/ 0// and 160 20/ 0//N and longitudes 320 42/ 0/ and 320 4/ 0// E. Study of the geological history of this area indicated that the repeated changes in the volcanic activity, alternated with destructive events, caldera collapses, produced a very complex system of basaltic and rhyolite rock masses. The variation in lithology, in the degree of tectonization and disturbance determined a wide spectrum of geotechnical materials, ranging from hard lavas to poorly welded pyroclastic deposits. Quarries, tunnels and other infrastructures were constructed upon these rock masses. This thesis is intended to investigate theoretically and experimentally the rock mass strength and deformability of these rocks in Sabaloka, in order to characterize them for engineering purposes. The estimate of rock mass strength and deformability is reasonably predicted through the use of empirical failure criteria such as the Hoek –Brown failure criterion which has gained broad acceptance in the rock mechanics community, and in situ tests and empirical expressions to predict deformability. The rock mass properties and modulus of deformations of these rocks have been carefully assessed based on laboratory tests (uniaxial compressive, tensile test, triaxial test), and field investigations. The rock mass characterization approaches, Rock Mass Rating (RMR), Rock Mass Quality (Q) and Geological Strength Index (GSI) systems have been applied in this thesis to predict and evaluate the rock mass properties and support design. Numerical modeling studies (RocLab and Examine 2 D programs) based on field and laboratory data, have been used to evaluate the performance of these rock masses. Tunnel stability problems were expected in both trachy basalt and vesicular basalt rock masses, hence, the support system was evaluated by means of Q-system which is the most proper one for support design of tunnels. The field and laboratory test results were analyzed, weighed and compiled together to reveal the engineering performance, of these different rock masses in term of strength and rock mass deformation modulus. The analyses of the results have shown that the investigated rock masses would be classified into three categories. Category I possesses very good strength with deformation modulus of 44115 MPa, uniaxial compressive strength of 164 MPa, Geologic Strength Index 72, Hoek constant mi , of 14.5, internal angle of friction Φ of 400 cohesion c 13 MPa, tensile strength of 1.37 MPa and rock mass strength of 56 MPa. Category II is of good quality with deformation modulus of 7515 MPa, uniaxial compressive strength of 96 MPa, Geologic Strength Index 45, Hoek constant mi, of 12.5, internal angle of friction Φ of 300 cohesion c 4.74 MPa, tensile strength of 0.121 MPa and rock mass strength of 17 MPa. However, category III is of poor quality with deformation modulus of 586 MPa, uniaxial compressive strength of 48 MPa, Geologic Strength Index 30, Hoek constant mi, of 8.8, internal angle of friction Φ of 240 cohesion c 1.6 MPa, tensile strength of 0.028 MPa and rock mass strength of 5 MPa.